He continued, "I didn't set out to be the first openly gay athlete playing in a major American team sport. But since I am, I'm happy to start the conversation."

Collins -- and his twin brother Jarron -- have been high profile athletes since high school ... when they were two of the highest rated prep players in the country.

They both went on to become standouts at Stanford ... and were each drafted into the NBA in 2001.

Jason says he didn't come out to his twin until last summer. "I'm glad I can stop hiding and refocus on my 13th NBA season."

NBA stars Kobe Bryant and Tony Parker are rallying behind Collins, tweeting messages of support. Kobe wrote, "Proud of @jasoncollins34. Don't suffocate who u r because of the ignorance of others." Tony added, "Really hope people will RESPECT Jason Collins for his decision to come out. Just glad he can now relax and not be afraid to be who he is."